1. Field of the Invention
This invention relates to an optical pickup apparatus that is used in a compact disc reproducing apparatus, a video disc reproducing apparatus, and the like.
2. Description of the Prior Art
An optical pickup apparatus that is used in compact disc reproducing apparatuses and the like uses a diffraction device or a holographic optical element, so that the number of components of the optical pickup apparatus can be reduced.
FIGS. 17 and 18 show such a conventional optical pickup apparatus, which comprises a light emitting device 31, a diffraction device 32, a collimating lens 33, an object lens 34, and photo-detectors 361 and 362. A light beam emitted from a light emitting device 31 first passes through a diffraction device 32 The diffracted beam of zero-order having thus passed through the diffraction device 32 is focused on the recording medium 35 through a collimating lens 33 and an object lens 34.
Then, the reflected beam from the recording face of the recording medium 35 again passes through the object lens 34, the collimating lens 33 and the diffraction device 32.
The beams reflected from the recording medium 35 pass through the object lens 34 and the collimating lens 33, and are diffracted by the diffraction device 32 The resulting first-order diffracted beams are introduced into the photo-detectors 361 and 362.
The diffraction device 32 is divided into two diffraction regions 32a and 32b by a division line that is parallel to the tracking direction. The first-order diffracted beam from the diffraction region 32a is focused on one photodetector 361 that is composed of photodetecting regions 36a and 36b and the first-order diffracted beam from the diffraction region 32b is focused on the other photodetector 362 that is composed of the photodetecting regions 36c and 36d. Output signals of the photodetecting regions 36a-36d are converted into a focus error signal FE, a tracking error signal TE and a reproduced information signal RF by a calculating circuit shown in FIG. 18. When the output signals of the photodetecting regions 36a-36d are represented, respectively, as Sa-Sd, the focus error signal FE is obtained by calculating (Sb+Sc)-(Sa+Sd) by means of summing circuits 37 and 38 and a subtracting circuit 39 based on the knife edge method The tracking error signal TE is obtained by calculating (Sc+Sd)-(Sa+Sb) by means of summing circuits 40 and 41 and a subtracting circuit 42 based on the push pull method The reproduced information signal RF is obtained by calculating (Sa+Sb+Sc+Sd) by means of summing circuits 40, 41 and 43.
In general, optical pickup apparatuses that are used in a compact disc reproducing apparatus and the like employ the three-spot method, in which two sub-beams for detecting a tracking error in addition to the main beam are used, in order to detect the tracking error signal TE.
On the contrary, the push pull method that is used in the above-mentioned conventional optical pickup apparatus is disadvantageous in that a displacement of the optical axis of an optical system from a given position causes an offset in the tracking error signal TE; for example, when the optical axis of the object lens 34 is displaced by a tracking servomechanism, the position of the peak of the intensity distribution of laser beams is displaced, as well, from the center of the optical axis. The push pull method is a method by which the light flux of these laser beams is divided into two portions that are along a division line parallel to the tracking direction and the tracking error signal TE is detected based on a difference in the intensity between these two light flux portions. Thus, a displacement of the peak position of the intensity distribution of the laser beams causes an offset in the tracking error signal TE.
In the above-mentioned conventional optical pickup apparatus using a diffraction device, an offset takes place in the tracking error signal TE due to a displacement of the optical axis of the optical system, which makes it impossible to achieve a precise tracking control.